The Class-2 and Class-3 are outer membrane proteins and are the meningococcal porins. They carry the serotype determinants of which there are over 20 in groups B and C. The aims of this study were to determine whether the N-terminal or the C-terminal of these proteins were surface exposed, and to locate the serotype determinants. Outer membrane vesicles (OMV) of M986 and M992 (Class-2) and C395 (Class-3) were digested with pronase. About half of class-2 protein from M986 and M992 were digested to yield a 38K membrane bound fragment. The Class-3 protein from C385 was resistant to this digestion. Amino acid sequencing of the class 2 38K fragment indicated that 25 N-terminal amino acids were surface exposed in both M986 and M992. It also shown that the first 50 amino acids of these 2 strains are identical. C-terminal specific carboxypeptidase Y failed to cleave either strain, indicating that this part of the protein is embedded in the outer membrane. Pronase digestion of the OMV did not change the reactivity of the M986 Class-2 protein with a serotype 2a monoclonal antibody. Thus, based upon these experiments it is evident that the exposed N-terminal does not carry the serotype determinant. Previous studies indicated that there are 5 prominent class 2 peptides exposed on the surface of the membrane, and 3 in case of Class-3. We plan to effect proteolytic or chemical cleavages that may alter the serotype specific determinants of the in situ proteins.